It seems like the mod causes an extreme amount of chunks being loaded in bigger forests.
Here is a warning message from my game log (These messages keep popping up when the server can't keep up.):

[03:04:21] [Server thread/WARN] [FML]: Dynamic Trees loaded a new chunk (11, -39  Dimension: 0) during chunk population, causing cascading worldgen lag. Please report this to the mod's issue tracker. This log can be disabled in the Forge config.

I think the warning from forge says more or less everything there is to say.

I'll work on a fix for this during the next dev cycle. It's a pretty harmless error since the cases when this occurs is infrequent and doesn't really cause true cascading chunk loading.

Then I've hit this case in my first world using this mod. Bad luck indeed (Took longer than 20 minutes to load, at which point I decided to stop the server).

Thanks for the reply and awesome mod!

I've made some bug fixes to my SimpleVoxelMap class that seem to have fixed this.. I've been flying through the world for nearly half an hour and I have not seen the lag warning. I'm calling it fixed in dev.